1. Field of the Invention
This invention relates to an engraver, and more particularly, to an engraving head having components which are easily mounted to the head and which are easily aligned with a plated cylinder to be engraved.
2. Description of Related Art
The basic principle of electro-mechanical engraving of a gravure cylinder involves rotating a plated cylinder while actuating an electrically driven tool which cuts or engraves cells or lines into the surface of the plated cylinder. The engraved cylinder is normally used in a web-type gravure printing press for printing paper, plastic, metallic film material, or other printed material.
In the gravure printing process, the engraved cylinder is flooded with ink, and a doctor blade wipes off excess ink from the surface so that only the engraved cells contain ink which is transferred to the material being printed. To obtain a high quality print, it is necessary that the cells be very accurately placed or located on the cylinder surface, usually within one or two microns of a desired predetermined location. The depth of the engraved cells must also be accurately controlled since the depth determines the amount of ink transferred which, in turn, determines the shade of gray in a black/white print, for example. In a color print, the amount of ink transferred to the paper or materials is even more critical since multiple colors are typically mixed to produce various shades of all possible colors. A slight variation in the desired amount of ink effects not only the color, but, more importantly, the density of the desired color.
In order to properly control the depth of the cells, the relative location between the plated cylinder and the electrically driven tool, which is typically a diamond stylus, must be accurately controlled. In order to ensure that the stylus is maintained at a constant distance from the plated cylinder as the cylinder rotates in an engraving process, the engraving head containing the stylus is provided with a diamond shoe tool which contacts the cylinder to maintain a desired predetermined spacing between the stylus and the cylinder. In the past, a diamond shoe tool has been used incorporating either a single ball-shaped engaging surface or a flat diamond engaging surface which requires precise tangential alignment relative to the cylinder surface. The ball-shaped diamond shoe tool provides a relatively small radius providing a small contact area between the shoe tool and the cylinder such that the ball shoe tool typically follows the irregularities in the cylinder surface well. However, this small contact area also results in a relatively high force being applied at the point of contact and, as a result, a ball shoe tool cannot be used for multiple pass engraving processes in that the shoe tool tends to distort or otherwise damage the cell walls created by the stylus on earlier passes. The flat diamond shoe tool is normally formed with a size sufficient to permit a better distribution of forces. However, although the flat shoe tools provide better distribution of forces applied to the cylinder than ball shoe tools, the flat shoe design often does not intimately follow the contours of the cylinder surface.
The placement of the diamond stylus within the engraving head of the engraver is also critical to proper formation of the engraved cells. In the past the stylus was mounted to a tool holder which was then located within an elongated aperture formed in a stylus arm wherein the tool holder would be manually positioned at a selected location within the aperture to cause the stylus to project outwardly a desired distance. Once the stylus was located in the desired position, fasteners threaded into the stylus arm were moved into engagement with a clamp member which would then cause the tool holder to frictionally engage the walls defining the aperture in the stylus arm. Typically, the tool holder would be formed with a D-shaped cross section having a flat side and a curved side, wherein the clamping member would engage the flat side of the tool holder and the curved side thereof would be forced into engagement with a cooperating curved surface of the aperture in the stylus arm. Thus, mounting of the stylus within the stylus arm required an operator to perform an alignment procedure in order to ensure that the placement of the stylus within the stylus arm was accurate.
When the stylus exits an engraved cell, a burr of material is commonly created around the edges of the cell, and the burr material is removed by a diamond burr cutter mounted on the engraving head adjacent to the location of the stylus and the diamond shoe tool. Past burr cutters have comprised diamond tools having a cutting edge which was located over the center of an engraved cell or row of engraved cells to remove the burr material. The alignment between the burr cutter and the cylinder would vary with variations in the diameter of the cylinder such that it was necessary for an operator to perform an alignment operation to place the cutter in tangential engagement with the cylinder surface whenever the cylinder diameter was changed.
Accordingly, there is a need for an improved engraving head which provides for simplified mounting and alignment of the stylus within the engraving head, and including a simplified mounting for a self-aligning shoe which provides for improved alignment of the engraving head relative to the cylinder. In addition, there is a need for a burr cutter for use on an engraving head wherein the burr cutter may be used on a variety of cylinder diameters without requiring adjustment of the burr cutter to obtain tangential alignment with the cylinder.